Design of a high impedance MITL for RITS-3

RITS-3 is the three-cell, 4 MV, 156 kA, 70 ns embodiment of the full twelve-cell 16 MV Radiographic Integrated Test Stand (RITS) (Ian D. Smith et al., 2000). The well-instrumented RITS-3 experiments (David L. Johnson et al., 2002) now underway at Sandia are intended to investigate/validate critical design issues for scaling to RITS. These experiments use a magnetically-insulated transmission line (MITL) in which the increment in the operating impedance of the MITL from cell to cell is equal to the impedance of the individual pulse forming line (pfl)/induction cell (8 ohms). The matched load voltage that is obtained in this configuration is 4.0 MV and occurs when the load impedance equals the sum of the PFL impedances (24 ohms). This paper discusses the design of a higher impedance MITL intended to increase the RITS-3 output voltage from 4 MV to 5.25 MV for the same pulse forming line charging voltage. The fundamental operating impedance increment for the MITL steps is increased to 14.25 ohms providing a matched 5.25 MV, 123 kA, 70 ns output pulse. Particle-in-cell simulations (LSP) of the MITL power flow from the cell to the load predict a nominal output in agreement with the design value. The cathode (boundary) current and the vacuum flow (sheath) current from the simulations scale roughly as predicted by parapotential flow theory (M.Y. Wang et al., 1978). The increased cell voltage and core flux swing are well within the RITS-3 design levels. When the load impedance is <42.75 ohms the MITL behavior is divided into three separate phases. A vacuum precursor, a magnetic insulation phase when the operating impedance is determined by parapotential flow, and an over insulation phase where the impedance is determined by the load. The over insulation wave moves back up the MITL toward the source with a velocity of 0.3-0.6 the speed of light. The diode voltage is less than 5.25 MV and the boundary and sheath currents are observed to change significantly from those for a matched diode.